When manufacturing semiconductor devices and/or liquid crystal display devices, and the like, using lithography techniques, a projection-exposure apparatus is normally used. A projection-exposure apparatus uses an illumination-light flux (produced by a light source and passed through an illumination-optical system) to project and expose a pattern defined on a photomask or reticle (hereinafter referred to as a "mask") via a projection-optical system onto respective "shot fields" (individual exposure regions each representing one "die") on a photosensitive substrate such as a semiconductor wafer or glass plate, etc. (hereinafter referred to as a "substrate") to which a photosensitive agent, such as a photoresist, etc., has been applied. (A "die" represents the area occupied by one device on the surface of the substrate.) For exposure, the illumination-light flux is projected using a projection-optical system. The mask and substrate are each supported on a separate mask stage and substrate stage, respectively. Since the illumination-light source, illumination-optical system, and projection-optical system are normally stationary, at least the substrate stage is movable to permit exposure of various shot fields on the substrate wafer surface.
One type of conventional projection-exposure apparatus is the so-called "step-and-repeat" type. With such an apparatus, a photosensitive substrate is mounted on a substrate stage that can move in two dimensions. An operation is sequentially repeated in which the photosensitive substrate is "stepped" (moved a predetermined amount in a lateral direction relative to the optical axis of the projection-exposure apparatus) by the substrate stage. After each step, the entire pattern defined by the mask is exposed onto the respective shot field on the photosensitive substrate. Such an apparatus is normally a "reducing" type in which the image of the pattern as formed on the substrate is smaller than the pattern defined by the mask.
Another type of conventional projection-exposure apparatus is the so-called "step-and-scan" type, most of which are "reducing" (by which is meant that the image of the mask pattern formed on the substrate by the projection-optical system has a smaller surface area than the mask pattern). Such apparatus reducingly project the mask pattern onto each shot field on the photosensitive substrate by scanning, but "step" from one shot field to the next. In step-and-scan exposure apparatus, after the next shot field on the photosensitive substrate has been step-shifted into the exposure field of the projection-optical system, the mask and substrate must be synchronously moved in order to effect scanning exposure of the shot field. Such synchronous scanning requires that each of the mask and substrate stages accelerate from a stationary condition and move at a constant velocity. While the stages are moving at a constant velocity, a shutter in the illumination-optical system opens to allow the illumination-light flux to make the exposure.
The period beginning the instant that acceleration of the stages begins up to the moment that the shutter opens for an exposure is termed the "prescan period". During the prescan period, a check is automatically made of whether or not the mask and substrate are being synchronously scanned. A check is also made using a focal-position detection system normally present in such apparatus. The focal-point detection system is operable to place the surface of the shot field at the best-focus plane of the projection-optical system.
During the prescan period, errors are sometimes detected that are due, for example, to an error in acquiring focal-position data, or an imperfect synchronization of the positions or velocities of the mask stage and substrate stage. In the past, the incidence of such errors during the prescan period required that operation of the exposure apparatus be stopped, the error condition be evaluated and corrected, and scanning exposure subsequently restarted from the shot position at which the error occurred. However, since error evaluation and correction must be performed manually with such apparatus, such tasks are troublesome. Also, since operation of the exposure apparatus must be stopped in order to perform error evaluation and correction, the productivity of the exposure apparatus is decreased.